Smoothing the Transition from the Fixed Telephone Network
to the IP Network
Takayoshi Kagei

Deputy Director
Telecommunications Policy Division, Telecommunications Business Department
Telecommunications Bureau, Ministry of Internal Affairs and Communication

1. Introduction
In November 2015, due to declining numbers of subscriber
telephone contracts and the expectation that transit exchanges
and other such systems will reach their maintenance limits in or
around 2025, we announced our intention to migrate from a public
switched telephone network (PSTN*1) to an IP network. Under
NTT’s vision:• The core network (relay network) parts will be migrated to
IP networks (NGN*2) operated by NTT East and NTTWest.
• The access lines leading to user premises will continue to use
existing metal lines after the transition to IP networks, andexisting subscriber exchange equipment will be used as accesspoints for metal lines.
• While allowing users to continue using their existing phones,
we will also put forward the idea of providing new metal IP
phones.
Since the fixed communication networks of NTT East and
NTT West form Japan’s basic communication infrastructure,
we are obliged to follow the various systems currently prescribedby the Telecommunications Business Act. For this reason,

we consulted with the Information and Communications
Council in February 2016 on the subject of how to achieve asmooth transition in fixed telephone networks. Following thisconsultation, discussions were held at the Smooth Telephone
Network Transition Committee*3 across a wide range of issues
while conducting interviews with service providers and relatedorganizations. This committee compiled two reports, which were
published in March 2017 (“The post-transition IP network”) and
September 2017 (“Planning for a smooth transition”).
Based on these council reports, the Ministry of Internal
Affairs and Communications is developing a system to realize
a smooth transition to the IP network of the fixed telephone
network.
This article presents an overview of the main points of thesereports and the systems and facilities that will be affected, with aparticular focus on efforts related to the migration of facilities.

2. Key points of the Information andCommunications Council report
2.1 Basic philosophy concerning fixed telephones
In recent years, the number of metal phone (subscriber phone
and ISDN) subscribers*4 has been on a downward trend, but if■ Figure 1: Transition of fixed telephone network to IP network＊1: P ublic Switched Telephone Network.
＊2: N ext Generation Network.
＊3: T he committee is chaired by professor Hirotaka Yamauchi of the Graduate School of Commerce at Hitotsubashi University. This committee set up two working groups — one to study
telephone connection functions (headed by Professor Hitoshi Aida of the University of Tokyo Graduate School of Engineering), and another to study user protection (headed by Yoshinori Sakai
Professor Emeritus of Tokyo Institute of Technology and Associate Professor at the Open University of Japan) — and also conducted technical and special investigations.
＊4: N umber of subscriber phone contracts (including ISDN): 21.14 million (as of March 2017).

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■ Figure 2: Current status of fixed telephones (number of contracts and charges)

these are added to the growing number of IP phone (0AB–J IP
phone) subscribers,*5 then the total number of fixed telephone
subscribers in Japan is still over 55 million.
Today, a wide variety of means of communication are in anadvanced state of use, including mobile phones and broadband.
However, the fixed telephone network is available in every partof the country and provides a basic means of communication
to people’s homes, businesses, public buildings and the like,and therefore plays an important role as part of the essential
infrastructure of social and economic activity. This important role
will remain unchanged even after migrating to an IP network.
Furthermore, once fixed telephone services have transitioned
over to an IP network, it will be possible to charge a low tariff *6
for calls of any distance within Japan while ensuring that thecommunication quality is at least as good as what is currently
available, and it is expected that a range of new services will also
be made available to users by exploiting the characteristics of IP
networks such as their ability to perform data communication as
well as voice communication.
Based on this idea, NTT East and NTT West will startoffering metal IP phones instead of ordinary metal phones. Like
existing subscriber telephones, these metal IP phones are expected
to provide a universal service, and we should aim to maintain thesame standards of quality and reliability that people expect ofexisting metal phones.
2.2 Ensuring the quality and reliability of fixed telephones
Under the current system, communications carriers are obliged■ Figure 3: Metal IP phone charges and quality/reliability

to ensure their facilities comply with technical standards so thatusers can be provided with reliable and stable telecommunications
services.
With the transition from PSTN to an IP network, NTT
East and NTT West will provide new metal IP phones by
incorporating metal lines into NGN as well as optical lines, andthe telephone networks of each communications carrier will
be reconfigured so that their respective IP networks will beinterconnected (IP-IP connection) without passing through the
NTT East/West exchange system.
To adapt to these changes, we must ensure that the IP network
is able to provide lifelines in emergency situations and a reliable
and stable supply of fixed telephones to support everyday socialand economic activities. It is therefore necessary to draw up
technical standards for communication equipment to ensure thatit can provide the same standard of quality and reliability as theexisting telephone network.

＊5: Number of 0AB–J IP phone contracts: 32.41 million (as of March 2017).
＊6: N TT has announced plans for a flat rate of ¥8.5 per 3 minutes for metal IP phone calls to anywhere in Japan after the IP network transition. Currently, three-minute daytime calls to subscriber
phones in NTT East and NTT West are charged according to distance: ¥8.5 for calls in the same city, ¥20–40 for calls in the same prefecture, and ¥20–80 for calls to a different prefecture.

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2.3 Ensuring the ability of phones to connect
In the existing PSTN, the NTT East/West exchange system
serves as a hub that connects between the telephone networks ofeach communications carrier at points of interface (POIs) in eachof Japan’s administrative divisions.
With the IP network transition, the NTT East/Westexchange system will be abolished, the POIs will be consolidated,
and in principle every communications carrier in Japan will
establish IP-IP connections with either of two “connecting POIs”,
one in Tokyo and the other in Osaka. It has been confirmed thatthey will be allowed to share communication facilities (routers and
SIP servers), which will require the implementation of new callconnection functions.
It is expected that the connecting POIs used for IP-IP
connections between communications carriers will have dual
support for connections via L2 switches*7 and without L2 switches(using a patch panel*8 instead).
In addition to sorting out the role of these phone connection
functions, their connection rules and technical standards, we arearranging a scheme*9 whereby NTT East/West will maintain,

manage and operate the communication facilities and L2 switches
used by communications carriers inside connecting POI buildings.
2.4 Ensuring emergency call capabilities
Currently, in emergency calls, the PSTN line hold function
ensures that calls from metal lines are held unless the emergency
service disconnects, even if the caller drops the handset. On theother hand, a callback from the emergency service is required
when reporting from a mobile phone.
Since it will be difficult to implement a line hold function after
the IP network transition, NTT has proposed replacing it with acallback function. However, the emergency services have raised
issues with the speed and reliability of existing callback functions,so NTT must adequately demonstrate how functions equivalent
to those of current systems can be offered, or provide suitable
alternatives.
To address this situation, the Smooth Telephone Network
Transition Committee interviewed representatives fromemergency services and business operators including NTT, andexamined the functions of emergency calls after the IP network■ Figure 4: Phone network configuration after the IP network transition■ Figure 5: Emergency call functions compatible with the IP network transition＊7:	A type of device that relays data on a network. Specifically, a switch that acts as a relay by forwarding packets to other relays based on the MAC addresses in the packet headers.
(Called a Layer 2 switch because MAC addresses are handled by the second layer (data link layer) of the OSI reference model.)＊8 :	A panel used for accommodating and connecting a group of communication lines. Capable of selecting signal paths easily without using dedicated switching equipment.
＊9:	For L2 switches inside connecting POI buildings, communications carriers (consortiums) that want to use them enter into an IRU (indefeasible right of use) contract with NTT East/
West, and NTT East/West maintains, manages and operates the L2 switches.

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transition. As a result, it was concluded that the alternative
functions for line holding should have the following properties:• To facilitate callbacks from the emergency services to callers,
it is realistic and rational to implement five functions: (a)emergency service number notification,*10 (b) cancellationof call transfers,*11 (c) cancellation of call barring,*12 (d)restrictions on three-way calling,*13 and (e) priority callhandling in the event of a disaster.*14• Based on the growing proportion of mobile phone usage
in emergency calls, it makes sense to ensure that these fivefunctions are also implemented for mobile phones.*15
2.5 H
 ow to manage the allocation of numbers totelecommunications lines(1) Converting the number allocation mechanism for IP network
migration (IP-IP connection)
In the current PSTN, when a telephone call is made to anumber allocated to a user by NTT East/West, the call control
signals from the caller’s device (a metal telephone, optical IP
phone, or mobile phone) must always be connected to the NTT
East/West exchange system via the PSTN to provide a redirection
mechanism whereby communication can take place with the calldestination. However, this mechanism cannot deal with cases

where connections between the telephone networks of each carrier
are formed by IP-IP connections without involving the PSTN.
In an IP-IP connection, when a call is made to a numberthat all the carriers associate with a particular user, the calling
equipment uses a signal corresponding to an IP address to retrieve
the number from a number database belonging to the calling
carrier. This number database must have a mechanism that
performs a process called number resolution to connect the callwith its destination (ENUM system*16).
For this reason, to support the phased introduction of
IP-IP connections from January 2021, there is a need for the
introduction of systems such as obliging all carriers that originate
and/or receive calls to set up a proper number database for number
management in IP-IP connections (including number resolution
by the ENUM system).
(2) Providing bidirectional number portability for fixed phones
In the current fixed phone system, number portability (theability to move telephone numbers) is only supported in “one way”
when migrating from an NTT East/West metal phone to a fixedphone of another carrier by signing up for a new contract, and is
not supported when migrating from an IP phone or from the fixedphone of another carrier besides NTT East/West.

■ Figure 6: Number management mechanism for IP-IP connections＊10:	Japan’s emergency services have so-called 1XY phone numbers (i.e., three digits starting with 1). This function reports the 1XY number of the corresponding emergency service backto the caller’s phone.
＊11:	A function that cancels the forwarding of incoming calls to a caller’s terminal, even if the caller is using a call forwarding function.
＊12:	A function that cancels the rejection of incoming calls to a caller’s terminal, even if the caller is using a call barring function.
＊13:	A function that temporarily restricts calls between the caller and any third party other than the emergency service.
＊14:	Currently, calls to the emergency services are handled with higher priority in the event of a disaster, but this priority treatment is not extended to calls made by the emergency
services back to the original callers.
＊15:	Since May 2017, with the aim of implementing these five functions in mobile phones, we have taken part in regular studies and discussions with a group of organizations including
the emergency services (police, fire and coastguard), three mobile communication companies (NTT DOCOMO, KDI and Softbank), and the Ministry of Internal Affairs andCommunications. A primary report detailing the results of these studies was reported to the Smooth Telephone Network Transition Committee on May 22, 2018.
＊16:	E.164 number mapping: a standard system that uses IP address query techniques to obtain information about the destinations to which phone numbers should be connected on theInternet.

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The proportion of numbers in the
fixed phone system (0AB–J) that cannot
have number portability has been on
an upward trend in recent years due
to progress in the IP transition, andis currently at 16%.*17 This figure is
expected to continue increasing in the
future.
Following the IP network transition,
all fixed-line users in the future will beusing IP phones. For the convenienceof these users, and in order to secure
a competitive infrastr uct ure for
fixed phones, we plan to introduce
bidirectional number portability by
the end of the IP network transition(January 2025). This will require somesystematic improvements.

■ Figure 7: Current status of number portability in fixed phones■ Figure 8: Principal forms of telecommunications number allocation and usage(3) Ensuring fair and efficient use of
number resources
Currently, the national allocation
rate of mobile phone numbers startingwith 090/080/070 and toll-free numbers
starting with 0120 (i.e., the proportionof all numbers that have been madeavailable) is in excess of 90%, and it is
becoming difficult to deal with newallocation requests.
On the other hand, the actual number allocation rate of communications
carriers (the proportion of available
numbers that are currently in use) is about 70% for mobile phones
and about 55% for toll-free numbers. At the national level, about
90% of available toll-free numbers have been assigned to a single
carrier.
Under these circumstances, the current system allows carriers
to voluntarily return numbers that have been allocated to them,regardless of whether or not these numbers are in use (voluntary
notification). As a result, numbers that have remained unused forlong periods of time are not being returned, leading to problems
such as inability to manage or redistribute numbers flexibly.
To adapt to the different situation regarding number
management brought about by the IP network transition (IP-IP
connection), it will be necessary to set up systems for dealing withthis shortage of numbers and the issues of unused numbers, andfor implementing the fair and efficient usage and redistribution offinite and scarce number resources. These include:• A mechanism for regularly confirming the state of useof telecommunications numbers and the discharge ofobligations related to telecommunications numbers• A mechanism that enables the disposal of numbers, such as
the cancellation of telecommunications numbers that havebeen left unused for a certain period, or the cancellation of

numbers when obligations relating to the handling of thesenumbers have not been not fulfilled
2.6 Road map for facility migration
The migration from PSTN to IP networks is to proceed
according to the following stages and schedule:• Stage 1: F ollowing preparator y work (development,
verification, etc.) by communications carriers,
establish a connecting POI building environmentfor IP-IP connections between carriers (by January
2021)• Stage 2: Connect the subscriber exchange systems of NTT
East/West to the IP network (NGN), and gradually
implement IP-IP connections between optical IP
phones and other carriers, and IP-IP connections
between other carriers (from January 2021 toJanuary 2024)• Stage 3: Implement IP-IP connections between NTT East/
West metal IP phones and other carriers (fromJanuary 2024 to the completion of transition in
January 2025)(Note: The migration of services from subscriber telephones to＊17:	As of March 2017, there are currently 9.97 million fixed phone numbers that do not have number portability. This constitutes 16% of all phone numbers (62.43 million), a proportion
that has been increasing in recent years.

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■ Figure 9: Migration of facilities from PSTN to the IP network■ Figure 10: Facility migration road map

metal IP phones (with a simultaneous exchange of contracts) was
conducted in January 2024)
To ensure that calls are handled properly during this transition
process, it is necessary to establish a telecommunications number
system and other such technical standards to ensure there is afunctioning number management mechanism that is compatible
with IP-IP connections.

3. System revisions to promote the smoothmigration of facilities (revision of theTelecommunications Business Act)
Based on a report by the Information and Communications
Council, the Ministry of Internal Affairs and Communications

has conducted a study of systematic measures. On March 6, 2018,a report on partial revision of the Telecommunications BusinessAct and the NICT Act was presented at the 186th Diet session,
and was established in law on May 16th and promulgated on May
23rd of the same year.*18
According to this law, from the perspective of promoting asmooth migration of facilities from PSTN to the IP network,
efforts are being made to switch to a mechanism where theresponsibility for number management is borne by all
carriers involved in the IP network transition, and to set up
telecommunications number systems that address the urgent
need for a larger pool of numbers to meet the needs of growth in
the mobile and IoT sectors. The points that were changed in the＊18:	The enforcement date of the Act is regarded as the day specified by a Cabinet Order within a period not exceeding one year from the date of promulgation.

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■ Figure 11: A mendment of the Telecommunications Business Act(improvement of rules relating to telecommunication numbers)

revised items were as follows:• T he Minister for Internal Affairs and Communications
shall draw up and publish a telecommunications number
plan (hereinafter referred to as “number plan”) including thecontents of telecommunications duties to be provided foreach type of telecommunications number, the conditions forusing these numbers (handling important communications,
supporting bidirectional number portability, expiration
dates, etc.), and limits on how many numbers can be
specified.
• Communications carriers who intend to use telecommunications numbers for the provision of telecommunications
services must prepare a telecommunications number usage
plan (hereinafter referred to as a “usage plan”) and obtaincertification from the Minister for Internal Affairs andCommunications.
• T he Minister for Internal Affairs and Communications
shall examine the usage plans prepared by telecommunications carriers in terms of their suitability based on thenumber plan, etc., and shall allocate telecommunications
numbers after approving the usage plan.*19• Telecommunications carriers must use the telecommunications numbers designated by the Minister for Internal Affairs
and Communications in accordance with the usage plan
approved by the Minister for Internal Affairs and Communications. If a violation occurs and a communications carrier
fails to comply with a compliance order from the Minister forInternal Affairs and Communications, this carrier’s certification is liable to be revoked.

4. Conclusion
The NTT East/NTT West PSTN is a core communication
infrastructure that acts as a hub to support phone calls andprovides a competitive foundation for many businesses to develop
their activities. Users and businesses will therefore be significantly
affected by the form taken by the network after the IP networktransition, and by the way in which this transition is achieved.
For this reason, prompted by the announcement of NTT’svision, we have spent about two and a half years actively engaged
in concrete discussions and studies with business operators,
enterprises and related organizations on a wide range of issues.
The Ministry of Internal Affairs and Communications has also
compiled a council report on how to implement the transition(together with a roadmap) based on about one and a half yearsof study, and has greatly accelerated the process by concentrating
its efforts on achieving a smooth transition, including systematic
improvements for greater effectiveness. Preparations for the
migration of facilities (including development and verification) are
now under way, and the actual transition process will start in 2021.
This transition process and the efforts of related organizations
will continue until we reach the milestones of service transition in
2024 and the completion of migration in 2025.
Now that we are approaching an important phase in thedevelopment of our communication infrastructure, we willcontinue to make every effort to support this initiative in our role
as a policymaker. Above all, we would like to express our sincere
gratitude for the great efforts and cooperation of everyone involvedin this initiative.
(Source of figures)• Figs. 1–10: S upplementary materials from reports by the
Information and Communications Council(“Ensuring a smooth transition of the fixed telephone
network”, published March 28, 2017 and September
27, 2017)• Fig. 11: Ministry of Internal Affairs and Communications＊19:	However, telecommunications carriers (MVNO, FVNO etc.) that do not assign numbers to users upon designation of a telecommunications number have established the same usage
plan as the standard telecommunications number usage plan stipulated by the Minister for Internal Affairs and Communications , It is deemed to have been accredited by the
Minister of Internal Affairs and Communications.

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